This laboratory designed and synthesized the first biologically active Gamma-emitting steroids with the characteristics necessary for steroid receptor studies. These steroids, the C-16Alpha-halogenated analogs of estradiol were found to be chemically stable and to bind to the estrogen receptor with high affinity. 16Alpha-125I-Estradiol was synthesized in carrier free form (2,200 Ci/mmol) and is now generally used in many laboratories to measure estrogen receptors with great sensitivity. Analogs of 16Alpha-iodo-estradiol will be synthesized in attempts to produce estrogens that are more stable metabolically and thus suitable for in vivo imaging of estrogen responsive tumors. These compounds will be synthesized with 123I using techniques we have developed which allows rapid synthesis and purification of the Gamma-emitter. The 123I compound will be used for in vivo imaging, to detect estrogen receptor containing tumors in animal models and in humans. We have demonstrated that 16Alpha-125I-iodo-estradiol is specifically cytotoxic to estrogen sensitive cells. This effect is caused by destruction of DNA during the decay of 125I and requires the approximation of the hormone to DNA by the estrogen receptor. Further experiments will be performed to assess this cell killing in vitro under conditions where steroid metabolism, as well as DNA repair occurs. Kinetic studies will determine the dose relationship of the Gamma-emitter to cell killing and assess whether DNA repair inhibitors potentiate this effect. The 125I-labelled steroid will be used in an experiment to determine, in intact cells, the DNA binding region of the estrogen receptor in an estrogen responsive gene. Two steroids which bind with high affinity to the estrogen receptor and whose structures have the potential of alkylating agents, will be synthesized labelled with [3H], as affinity probes for the estrogen receptor. Two analogs of progestins labelled with 125I have already been synthesized and have been found to bind to the progesterone receptor with high affinity. These compounds will be synthesized carrier-free and their interaction with other steroid binding proteins and receptors will be measured. Analogs of glucocorticoids will be synthesized with 125I and tested as ligands for the glucocorticoid receptor.